Anyone can call themselves a 'professional photographer' and many people do, but it takes mad skills to really be a professional in the business of photography. I can go on for days about what makes a professional photograph, but for simplicity sake let's just start with one simple thing: A truly professional photograph will have detail in both the lightest and darkest parts of the image.
You hire a professional photographer to produce a 3-dimensional image that stands out on a 2-dimensional plane and shows the details and effectively communicates with the viewer. Sounds simple enough, but many self-taught 'professionals' haven't learned the basic first lesson; to capture detail in the highlights and the shadows.
The goal of a good photographer is to capture what he sees. Compared to the human eye, a camera does a very poor job of seeing. IF a photograph has no information or detail in the darkest areas of the image (novice) and/or no information or details in the lightest areas (Amateur) then it isn't a very professional photograph.
If someone is bragging to you about how 'professional' their photography is, just look at the details (or lack thereof) and you will know instantly how professional they are!